There are various ways to get a bootable live system on a usb pendrive (unetbootin, dd, isohybrid...) and usually involve formatting or overwriting the entire drive, i,e, data is destroyed. This is not always necessary.
All that's needed is a device with a valid mbr, a fat filesystem and syslinux configured. Multiboot is then possible. If the drive must be processed first standard Debian cli tools can do the job.
That done, Debian live-boot (even current sid) has some limitations that probably won't change anytime soon.
1. It can boot "fromiso" but the device (/dev/whatever) must be specified. That might be different on another machine. Otherwise you have to extract the squashfs.
2. The live-media partition gets mounted read-only. It must be large enough for one or more live images. You can't save data to it. You can't use it for persistence. Sure you can partition but then less usable RW space.
A named iso *can* be searched and found without specifying it's device (thanks GRML) and the live-media partition does *not* have to be RO !
I made a gui application (deb) for an initial usb setup, which will boot a Debian-Live system direct from ISO ("findiso"), with persistence and "live" write access, using a pendrive with a single FAT32 partition.
It does not alter any existing Debian live-boot files but generates a modified initrd for the live-boot media only. It can use a live iso on disk or use the squashfs while in a live session.
This (experimental) version is for Refracta:
http://www.exe-linux.fastfishwebsolutio ... .3_all.deb
Dependencies ex-current Refracta: pmount, fuseiso, pv
Testers appreciated, more later if anyone is interested. Other ideas for usb installs, multiboot methods or syslinux in general?